Addiction is a devastating disorder that affects over 20 million Americans. This disorder has recently been conceptualized as a pattern of maladaptive choice of drug rewards over non-drug alternative rewards. The factors that cause individuals to make such seemingly poor choices are not well understood. The goal of the research proposed here is to fill this critical gap in knowledge, in an effort to potentially help solve the problem of addiction. This will be accomplished by applying recent advances in the field of behavioral economics to an increasingly used choice-based animal model of addiction. Aim 1 is to investigate how the essential values of drugs (cocaine or heroin) and non-drug rewards (food or saccharin) determine the behavior of rats choosing between these rewards. Aim 2 is to investigate how the availability of each type of reward outside of the choice situation alters the essential values of drug and non-drug rewards and affects choice between them. Aim 3 is to determine how the frequency of choice opportunity influences the likelihood of the drug reward being chosen over the non-drug reward in different contexts of overall reward availability. Special focus will be placed throughout the proposed research on those situations hypothesized to promote excessive and maladaptive choice of the drug reward. Answering these questions will provide important information about the major factors that control the choice to use drugs, even when such drug use is detrimental to the long- term interests of the user. This information will be significant for the development of new strategies designed to prevent and treat drug addiction, a major public health problem.